1. Field of the Invention
The present invention relates to apparatus and methods for circularly polarized antennas and, more specifically, to an improved feed structure for supplying appropriately phased signals to each element of an array of microstrip radiating patches configured in a low loss unit cell configuration.
2. Description of Prior Art
Circularly polarized antennas have been found to be especially useful for space communications involving satellite transmissions. While linearly polarized antenna systems are very useful for many purposes, the constraints of a linearly polarized antenna system for space communications may require that an Earth station maintain a tight alignment with a satellite to achieve acceptable communications. Maintaining the tight alignment through the Earth's atmosphere is difficult especially when the receivers are located on ships, aircraft, and other objects that change position and orientation. Circularly polarized radiation is less affected by alignment considerations.
As another significant advantage for satellite communications, microstrip patch antennas are generally relatively light and small. Due to the high launch cost for each pound of weight and because of the limited cargo space for space launched packages, the small size and weight attributes of a microstrip patch antenna make this type of antenna especially suitable for satellite applications.
Typically, the circularly polarized microstrip antennas are used in arrays where numerous radiating elements are used. However, with an array of multiple microstrip patches comes an attendant need for a complex feed structure to supply each microstrip patch with a suitable phase excitation. Each microstrip patch requires a dual feed excitation that is preferably of equal amplitude and in phase quadrature. Moreover, each different patch will typically need to be fed with a different relative phase excitation as compared to adjacent patches. As well, it is desirable to have as little power loss as possible through the feed structure so that most radiation will occur from the antenna rather than from the feed structure itself. The layout of the feed structure should preferably be simple and avoid sharp corners. The following patents disclose various feed structures that show attempts to solve this difficult problem over the last two decades.
U.S. Pat. No. 5,661,494, issued Aug. 26, 1997, to P. K. Bondyopadhyay, is hereby incorporated herein by reference and discloses a microstrip antenna for radiating circularly polarized electromagnetic waves comprising a cluster array of at least four microstrip radiator elements. The dual fed circularly polarized reference element is positioned with its axis at a 45.degree. angle with respect to a unit cell axis. The other three dual fed elements in the unit cell are positioned and fed with a coplanar feed structure that results in sequential rotation and phasing. The centers of the radiator elements are disposed at the comers of a square with each side of a length d in the range of 0.7 to 0.9 times the free space wavelength of the antenna radiation. The radiator elements reside in a square unit cell area of sides equal to 2d and thereby permit the array to be used as a phased array antenna for electronic scanning and is realizable in a high temperature superconducting thin film material for high efficiency.
The present invention is especially suited for use with the exemplary unit cell configuration described in the above patent with respect to the disclosed microstrip patch positions and orientations. However, according to the present invention as discussed hereinafter, the feed lines to each patch are improved. Moreover, transmission line feed loss and circuit complexity is reduced according to the feed structure of the present invention.
U.S. Pat. No. 4,543,579, issued Sep. 24, 1985, to T. Teshirogi, discloses a circular polarization antenna having wide-band circular polarization characteristics and impedance characteristics is accomplished by feeding N-antenna elements which are shifted at an interval of .pi./N radians with respect to the boresight direction with a differential phase shift of an interval of .pi./N radians corresponding to the angular orientation of the antenna elements so as to obtain circular polarization with respect to the boresight direction.
U.S. Pat. No. 5,231,406, issued Jul. 27, 1993, to A. I. Sreenvas, discloses a circular polarization antenna wherein signals are fed to an array of electromagnetically coupled patch pairs arranged in sequential rotation by and interconnect network which is coplanar with the coupling patches of the patch pairs. The interconnect network includes phase transmission line means, the lengths of which are preselected to provide the desired phase shifting among the coupling patches.
U.S. Pat. No. 4,191,959, issued Mar. 4, 1980, to J. L. Kerr, discloses a microstrip antenna having an etched metal radiator element including a polarizing patch consisting of a two-dimensional removal of metallization from the central portion of the radiator element with one dimension of the polarization patch being greater than the other dimension, e.g., an elongated rectangle and selectively oriented with respect to the input axis whereby, for example, circular polarization is achieved by means of orienting the polarization patch substantially 45.degree. with respect to the input axis.
U.S. Pat. No. 4,713,670, issued Dec. 15, 1987, to Makimoto et al., discloses a microwave plane antenna comprising a plurality of pairs of antenna elements connected at their one end to a power supply circuit and respectively including at the other terminating end an impedance-matched patch antenna means, whereby signal energy remaining at the terminating ends of the antenna elements is caused to be effectively utilized as radiation energy, and any power loss is restrained for a high antenna gain and improved aperture efficiency.
U.S. Pat. No. 4,943,809 and U.S. Pat. No. 4,761,654, issued Jul. 24, 1990 and Aug. 2, 1988, to A. L. Zaghloul, disclose a microstrip antenna array having broadband linear polarization, and circular polarization with high polarization purity, feed lines of the array being capacitively coupled to feed patches at a single feedpoint or at multiple feedpoints, the feeding patches in turn being electromagnetically coupled to corresponding radiating patches. The contactless coupling enables simple, inexpensive multilayer manufacture.
U.S. Pat. No. 4,973,972, issued Nov. 27, 1990, to J. Huang, discloses a circularly polarized microstrip array antenna utilizing a honeycomb substrate made of dielectric material to support on one side the microstrip patch elements in an array, and on the other side a stripline circuit for feeding the patch elements in subarray groups of four with angular orientation and phase for producing circularly polarized radiation, preferably at a 0.degree., 90.degree., 180.degree., and 270.degree. relationship. The probe used for coupling each feed point in the stripline circuit to a microstrip patch element is teardrop shaped in order to introduce capacitance between the coupling probe and the metal sheet of the stripline circuit that serves as an antenna ground plane. The capacitance thus introduced tunes out inductance of the probe.
U.S. Pat. No. 4,833,482, issued May 23, 1989, to Trinh et al., discloses an antenna arrangement for radiating and receiving circularly polarized radiation. A first antenna array having parallel stripline conductors is disposed on the top surface of a dielectric substrate. The stripline conductors have radiating tabs protruding outwardly therefrom in a direction of about forty-five degrees from the stripline conductors. A second antenna array having a second plurality of stripline conductors are interdigitated with the first stripline conductors.
Those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.